Synergistic Noise-By-Flow Control Design of Blade-Tip in Axial Fans: Experimental Investigation

The increasing concern on noise emission had recently inspired the definition of a regulatory framework providing standards on eco-design requirements for energy-using products and noise levels, i.e. the European Directive 2005/32/EC (European Parliament, 2005). The compliance to standards within the fan industry appears more stringent than for other sectors because of their use in ventilation systems entailing the direct human exposure to noise emission. This driver demands the elaboration of fan design solutions and technologies in which the exploitation of noise control strategies must not affect aerodynamic performances. To summarise, the main generation mechanisms of aerodynamic noise in low-speed axial fans are: turbulent inflow, self noise (turbulent or laminar boundary layers, boundary layer separation), trailing edge noise, secondary flows (Fukano et al., 1986), and tip leakage related noise. Among these aerodynamic phenomena, Inoue and Kuroumaru (1989), Storer and Cumpsty (1991) and Lakshminarayana et al. (1995) investigated the role of tip leakage flows in compressor rotors and demonstrated the three-dimensional and unsteady nature, and the influence on aerodynamic losses and noise generation. In the context of low-speed turbomachines, Akaike et al. (1991) pointed out that the vortical structure near the rotor tip in industrial fans is one of the major noise generating mechanisms. The tip leakage noise can be one of the most significant sources correlated to the broadband spectral signature (Longhouse, 1978; Fukano et al., 1986; Kameier & Neise, 1997). Kameier and Neise (1997) highlighted that, in addition to the broadband influence, tip leakage flows could be responsible for narrowband tones at frequencies below the blade passing frequency in coincidence with a tip vortex separation. During the last decades, noise control has emerged as a new field of research as the literature demonstrates (Gad-el-Hak, 2000; Joslin et al., 2005), and scholars have proposed noise reduction strategy classifications, which distinguish between passive, active and reactive devices. A number of research programmes envisioned designs tailored to a synergistic noise and flow control by incorporating structural (passive) or flow (active)